1. Field of the Invention
The present invention relates to a method of making an improved carbon foam material and particularly such a method which involves treatment of a carbon foam precursor followed by a low pressure foaming. In another embodiment of the invention, a carbonaceous feedstock is subjected to oxidation to create a partially devolatilized carbon precursor extract and subsequent solvent treatment to dissolve at least a portion of the hydrocarbon molecules and, thereby, facilitate production by coking of a carbonaceous material having a high density.
2. Description of the Prior Art
It has been known for many decades that coal can be beneficiated for application in a wide variety of environments. For example, it has been known that coal may be employed as a fuel in electric utility plants and, in respect of such usages, beneficiating of the coal will reduce the ash content and the amount of sulfur and nitrogen species contained in the gaseous exhaust products.
It has also been known to convert coal into coke for use in various process metallurgy environments.
It has also been known to create carbon foam materials from feedstocks other than coal, which can be glassy or vitreous in natures and are brittle and not very strong. These products which lack compressive strength tend to be very brittle and are not graphitizable. See, generally, Wang, “Reticulated Vitreous Carbon—A New Versatile Electrode Material,” Electrochimica Acta, Vol. 26, No. 12, pp. 1721-1726 (1981) and “Reticulated Vitreous Carbon An Exciting New Material,” Undated Literature of ERG Energy Research and Generation, Inc. of Oakland, Calif.
It has been known through the analysis of mechanical properties of carbon fibers that long-range crystallite orientation is achieved by alignment of the precursor molecules during fiber spinning. In “Idealized Ligament Formation in Geometry in Open-Cell Foams” by Hager et al., 21st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 102-103 (1993), a model analysis regarding interconnected ligament networks to create geometric evaluation of hypothetical ligamentous graphitic foam is disclosed. This model analysis, however, does not indicate that graphite foam was made or how to make the same.
It has been suggested to convert synthetic naphthalenic mesophase pitch into a carbon foam product by employing a blowing/foaming agent to create bubbles in the material, followed by graphitization of the resultant carbonized foams above 2300° C. See “Graphitic Carbon Foams: Processing and Characterizations” by Mehta et al., 21 st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 104-105 (1993). It is noted that one of the conclusions stated in this article is that the mechanical properties of the graphitic cellular structure were quite low when compared to model predictions.
It has been known to suggest the use of graphitic ligaments in an oriented structure in modeling related to structural materials. See “Graphitic Foams as Potential Structural Materials,” Hall et al., 21st Biennial Conference on Carbon, Conf. Proceedings, American Carbon Society, Buffalo, N.Y., pp. 100-101 (1993). Graphitic anisotropic foams, when evaluated mathematically in terms of bending and buckling properties, were said to have superior properties when compared with other materials in terms of weight with particular emphasis on plate. structures. No discussion of compressive properties is provided.
In “Carbon Aerogels and Xerogels” by Pekala et al., Mat. Res. Soc. Symp. Proc., Vol. 270, pp. 3-14 (1992), there are disclosed a number of methods of generating low-density carbon foams. Particular attention is directed toward producing carbon foams which have both low-density (less than 0.1 g/cc) and small cell size (less than 25 microns). This document focuses upon Sol-gel polymerization which produces organic-based Aerogels that can be pyrolyzed into carbon Aerogels.
In “Carbon Fiber Applications,” by Donnet et al., “Carbon Fibers,” Marcel Decker, Inc., pp. 222-261 (1984), mechanical and other physical properties of carbon fibers were evaluated. The benefits and detriments of anisotropic carbon fibers are discussed. On the negative side are the brittleness, low-impact resistance and low-break extension, as well as a very small coefficient of linear expansion. This publication also discloses the use of carbon fibers in fabric form in order to provide the desired properties in more than one direction. The use of carbon fibers in various matrix materials is also discussed. A wide variety of end use environments, including aerospace, automotive, road and marine transport, sporting goods, aircraft brakes, as well as use in the chemical and nuclear industries and medical uses, such as in prostheses, are disclosed.
It has been known to make carbon fibers by a spinning process at elevated temperatures using precursor materials which may be polyacrylonitrile or mesophase pitch. This mesophase pitch is said to be achieved through conversion of coal tar or petroleum pitch feedstock into the mesophase state through thermal treatment. This thermal treatment is followed by extrusion in a melt spinning process to form a fiber. The oriented fiber is then thermoset and carbonized. To make a usable product from the resulting fibers, they must be woven into a network, impregnated, coked and graphitized. This involves a multi-step, costly process. See “Melt Spinning Pitch-Based Carbon Fibers” by Edie et al., Carbon, Vol. 27, No. 5, pp. 647-655, Pergamen Press (1989).
U.S. Pat. No. 5,705,139 discloses a method of employing the solvent extraction process of U.S. Pat. No. 4,272,349 as a basis for selecting a particular coal extract of bituminous coal to produce isotropic coke and graphite from solid extracts obtained by non-destructive solvent treatment of coal. These materials are all solid. The patent also acknowledges the existence of petroleum pitch which is said to have certain prior uses in respect of a binder pitch, as well as possible use as a raw material for graphite or other carbon articles, but is said to suffer the same disadvantages as petroleum coke.
It has been known to heat carbonaceous feedstocks to create coke at temperatures of about 350° C. to 800° C. depending on the nature of the carbonaceous material. The heat serves to drive off the volative gases leaving behind a carbon residue which is referred to as “coke”. Coke usually has a certain degree of porocity as outgassing volatile gases form bubbles. Highly porous coke is generally referred to as “foam”.
U.S. Pat. No. 3,960,770 discloses a means of producing a porous coke employing a polymer precursor.
U.S. Pat. No. 5,626,977 discloses a method of using carbon aerogels to create carbon foams said to be suitable for use as electrodes.
U.S. Pat. No. 6,323,160 discloses a product for densifying carbon foam using carbonaceous material as a filler material.
U.S. Pat. No. 3,917,806 discloses a specific preferred means of preparing a porous carbon molding having a low density which involves mixing a specific pitch with at least one aromatic hydrocarbon and immersing the mixed material in a solvent in which the pitch is not highly soluble, but the aromatic hydrocarbon is, followed by calcining the infusible porous material. The product is created by etching action rather than volatilization.
U.S. Pat. No. 4,025,689 discloses the use of a carbonaceous substance which may be petroleum coke, pitch coke, graphite, coal, charcoal or carbon black in making a graphitized hollow sphere wherein a foamed polystyrene coated with a carbonaceous powder and binder are heated to volatilize the core in making the porous article.
U.S. Pat. No. 4,289,604 discloses a method for manufacturing isotropic coke from a tar or tar pitch.
U.S. Pat. No. 5,888,469 discloses methods of making either anisotropic carbon foam material or isotropic carbon foam material employing coal as a starting material with processing in one embodiment including hydrogenating the coal followed by de-ashing, separation of asphaltenes, coking and graphitizing and in another following a similar process, but not involving hydrogenating the coal.
There remains, therefore, a very real and substantial need for an improved method of making a carbon foam product which involves specific treatment of a carbon foam precursor so as to facilitate creating a foam material with low pressure processing. There is also a need for a method of pretreating a carbonaceous feedstock to create a partially devolatilized carbon precursor extract which will be treated by solvent and thereby facilitate coking.